Transformation And Dynamics Between Magnetic Skyrmions And Bobbers In Stepped Nanostructures

Magnetic skyrmion and bobbers are two types of different nanoscale spin configurations that can coexist in nanostructures of the chiral magnets.We then proposed to utilize these two magnetic topologic structure as binary bits "1" and "0"respectively to build racetrack memory devices,in order to solve the issue of easy data loss from the purely skyrmion based racetrack memory device.Creating magnetic bobbers and skyrmions controllably and driving their motions are the two prerequisites to realize the device.Here,using micromagnetic simulations,we demonstrate that a magnetic bobber and a skyrmion tube can be transformed to each other and driven together using spin-polarized current in nanostripes,we also showed such stepped nanostructures can be readily applied as the write head for the skyrmion-bobber-based racetrack memory.The main results are shown below:(1)We designed a stepped FeGe nanostructure to realize a inter transformation between magnetic skyrmions and bobbers.When the spin polarized current is used to drive a skyrmion tube in thin region into thick region,it is possible to create a magnetic bobber.During this process,we found that a skyrmion tube can successfully turn into a bobber only when the thickness of thin region is in between 72nm and 81nm.The critical current is about 5.0×1011Am-2.(2)we investigated the current-induced dynamics of magnetic skyrmions and magnetic bobbers.When a DC is used,the simulated results show the speed of magnetic skyrmion is proportional to the current density,while the magnetic bobber is easily blocked.The blocked phenomenon of magnetic bobbers will disappear when a pulse current is used.Based on this,we show the magnetic skyrmion and magnetic bobber can move collectively stimulated by the current pulses.We further demonstrate the possibility to use skyrmion tube and bobber as data bit to build the skyrmion-bobber-based racetrack memory device.